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| United States Patent |
5,574,758
|
|
Murakami
, et al.
|
November 12, 1996
|
Method for measuring gamma-rays of radionuclides, particularly in
primary water of nuclear reactor
Abstract
A gamma-rays measurement method of radionucludes (iodine-131, cobalt-60,
etc.) coexisting with radionuclides (nitrogen-13, fluorine-18, cobalt-58,
etc.) each emitting a pair of annihilation gamma-rays, particularly in
primary water of a nuclear reactor by the use of a gamma-ray spectrometric
system which includes a primary detector for detecting the gamma-rays and
the one annihilation gamma-rays as pulses, a secondary detector for
detecting the other annihilation gamma-rays as pulses, and shield detector
for detecting gamma-rays Compton-scattered and escaped from the primary to
shield detectors as pulses. The method comprises counting the pulses of
the secondary detector in anticoincidence with the pulses of the primary
detector, thereby to reject the recording of the annihilation gamma-rays
from the primary detector, thus minimizing the annihilation gamma-rays
disturbing to the measurement, followed by determining count numbers of
the gamma-rays. Detection limits of the gamma-rays can be elevated
significantly thereby. Simultaneously, further anticoincidence counting of
the shield detector with the primary detector can be conducted to diminish
the Compton-scattered gamma-rays.
| Inventors:
|
Murakami; Ryuji (Nara, JP);
Yamada; Masataka (Osaka, JP);
Shintani; Hirofumi (Osaka, JP);
Ando; Shingo (Kanagawa, JP)
|
| Assignee:
|
Nuclear Engineering, Ltd. (Osaka, JP)
|
| Appl. No.:
|
371291 |
| Filed:
|
January 11, 1995 |
Foreign Application Priority Data
| Current U.S. Class: |
376/245; 250/370.06; 250/371; 376/253 |
| Intern'l Class: |
G21C 017/02 |
| Field of Search: |
376/245,250,251,253
250/303,370.06,370.11,370.07,371
|
References Cited
U.S. Patent Documents
| 3783268 | Jan., 1974 | Neeb et al. | 250/83.
|
| 3786257 | Jan., 1974 | Weiss et al. | 250/83.
|
| 3819936 | Jun., 1974 | Weiss et al. | 250/83.
|
| 3849655 | Nov., 1974 | Martucci | 250/366.
|
| 4532103 | Jul., 1985 | Kitaguchi et al. | 376/245.
|
| 4841153 | Jun., 1989 | Wormald | 250/390.
|
Other References
Nuclear Instruments and Methods, vol. 133, No. 1 (1976), pp. 35-49, Stelts
et al.
Proc. of Conf. on Semiconductor Nuclear-Particle Detectors and Circuits.
Gatlinburg, Tenn. USA, (1968) pp. 693-702, Camp.
|
Primary Examiner: Behrend; Harvey E.
Attorney, Agent or Firm: Flynn, Thiel, Boutell & Tanis, P.C.
Claims
What is claimed is:
1. A method for measuring selectively gamma-rays of radionuclides, in
primary water of a nuclear reactor, which further contains radionuclides
each emitting a pair of annihilation gamma-rays in diametrically opposite
directions, by the use of a gamma-ray spectrometric system which includes
a primary detector for detecting photons of the gamma-rays and photons of
the one annihilation gamma-rays in the one direction, a secondary detector
for detecting at least photons of the other annihilation gamma-rays in the
opposite direction, a shield detector for detecting Compton-scattered
photons of the gamma-rays escaped from the primary detector to the shield
detector, and an anticoincidence circuit connecting with the primary,
secondary, and shield detectors, the primary detector and the secondary
detector being located in opposed manner relative to the axis of a coolant
pipe through which the primary water flows, the shield detector
surrounding the primary detector except for its portion facing the pipe on
which the gamma-rays and the annihilation gamma-rays are incident, which
method comprises: detecting the photons of the gamma-rays and the photons
of the one annihilation gamma-rays on the primary detector as pulses while
detecting the photons of the other annihilation gamma-rays on the
secondary detector as pulses;
counting the pulses of the secondary detector in anticoinci- dence with the
pulses of the primary detector thereby to reject the recording of the
pulses of the annihilation gamma-rays from the primary detector, thus
minimizing the annihilation gamma-rays on the primary detector; and
subsequently determining count numbers of the gamma-rays.
2. A method for measuring selectively gamma-rays of radionuclides, in
primary water of a nuclear reactor as set forth in claim 1, which further
comprises simultaneously detecting the Compton-scattered photons of the
gamma-rays on the shield detector as pulses; and counting the pulses of
the shield detector in anticoincidence with the pulses of the primary
detector thereby to reject the recording of the Compton-scattered
gamma-rays from the primary detector, whereby the Compton-scattered
gamma-rays on the primary detector are also diminished.
3. The method for measuring selectively gamma-rays of radionuclides as set
forth in claim 1, which comprises using a semiconductor detector or a
scintillation detector as the primary detector, and scintillation
detectors as the secondary and shield detectors.
4. The method for measuring selectively gamma-rays of radionuclides as set
forth in claim 2, which comprises using a semiconductor detector or a
scintillation detector as the primary detector, and scintillation
detectors as the secondary and shield detectors.
5. A method for measuring selectively gamma-rays of radionuclides as set
forth in claim 3, wherein the semi-conductor detector is selected from a
germanium detector, lithium drift silicon detector and cadmium telluride
detector; and the scintillation detector is selected from a detector of
sodium iodide activated by thallium, detector of cesium iodide activated
by thallium, and bismuth germanate detector.
6. A method for measuring selectively gamma-rays of radionuclides as set
forth in claim 4, wherein the semi-conductor detector is selected from a
germanium detector, lithium drift silicon detector and cadmium telluride
detector; and the scintillation detector is selected from a detector of
sodium iodide activated by thallium, detector of cesium iodide activated
by thallium, and bismuth germanate detector.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a method for measuring gamma rays of trace
amounts of radionuclides (radioisotopes), such as iodine-131, cobalt-60,
etc., particularly in primary water of a nuclear reactor, which further
contains other radionuclides such as nitrogen-13, fluorine-18, cobalt-58,
etc. each emitting a pair of annihilation gamma-rays. More particularly,
the invention is concerned with an improved gamma-ray spectrometric method
for measuring selectively gamma-rays of the aforementioned radionuclides
by diminishing the annihilation gamma-rays emitted by radionuclides
coexisting in the primary water, thus significantly elevating the
detection limits of the gamma-rays.
2. Statement of Related Art
For instance, at nuclear power plants, with a view toward safe operation
thereof, leakage of nuclear fuel assemblies is always kept under
surveillance, for example, by measuring gamma-rays of .sup.131 I .sup.60
Co ,etc. contained in trace amounts in water of a primary coolant of an
individual nuclear reactor.
However, other radionuclides such as .sup.13 N, .sup.18 F, .sup.58 Co,
etc., which are unstable radionuclides emitting .beta..sup.+(e.sup.+) or
positron, are also contained in the primary water, and soon .beta..sup.+
decay at a low energy level by absorption in a substance and bonding with
electrons therein at the end of their ranges. At that time, one positron
and one electron are annihilated, emitting annihilation gamma-rays of
0.511 MeV in diametrically opposite directions.
The coexistence of the radionuclides emitting the annihilation gamma-rays
is a major disturbing factor for the measurement of the intended
gamma-rays, particularly gamma-ray of .sup.131 I, which has a close energy
level (0.364 MeV) to the annihilation gamma-rays.
Additionally, Compton scattering caused inevitably in a gamma-ray
spectrometry also interferes with the intended measurement.
As a consequence, it is essential for gamma-ray spectrometric measurement
of gamma-rays of the intended radionuclides (.sup.131 I, .sup.60 Co, etc.
) in the primary water that the annihilation gamma-rays be minimized while
Compton backgrounds or continua of the resulting gamma-spectra due to the
gamma-rays and annihilation gamma-rays are suppressed.
Hitherto, iodine-131 and other radionuclides emitting gamma-rays in water
of a primary coolant has been measured by means of a germanium (Ge)
detector or a scintillation detector of NaI(T1) (sodium iodide activated
by thallium) or Bi.sub.4 Ge.sub.3 O.sub.12 (bismuth germanate known as
BGO), or a gamma-ray sepctrometric measurement system wherein a
scintillation detector is disposed around a germanium detector. The method
using the Ge detector was poor in detection limit of .sup.131 I owing to
the effect of Compton backgrounds produced from .sup.131 I gamma-ray,
.sup.60 Co gamma-ray the annihilation gamma-rays, etc., so that trace
amounts of .sup.131 I and other radionuclides emitting gamma-rays in the
primary water couldn't be measured. Only in the event that .sup.131 I,
.sup.60 Co, etc. were leaked from a fuel assembly into the primary water,
increased concentrations of them enabled the measurement.
Again, the method using the NaI(T1) detector was too inferior to the Ge
detector method in resolution power.
The method using both Ge detector and scintillation detector has been
improved more or less over the preceding methods, but it has still not
been possible to measure extremely slight concentrations of .sup.131 I,
.sup.60 Co and others.
Thus, any of the known gamma-ray spectrometric methods has not been
satisfactory and feasible because the annihilation gamma-rays from
coexsiting radionuclides in the primary water have interfered with the
measurement of the intended radionuclides, e.g., .sup.131 I, etc.
Another method for measuring .sup.131 I and other radionuclides by chemical
analysis has been known, but has yielded disadvantageously awkward
radioactive wastes, which should be handled or disposed of with great
care. Hence, this is not suitable for frequent or continuous measurement.
In view of the drawbacks or problems as encountered in the prior art
measurement methods of gamma-rays in primary water of a nuclear reactor as
stated above or gamma-rays in another radioactive substances, the present
invention is designed to provide a gamma-ray spectrometric measurement
method which enables to significantly enhance detection limits of
gamma-ray-emetting radionuclides, particularly in the primary water.
That is to say, it is a primary object of the invention to provide an
improved method for measuring selectively gamma-rays of radionuclides
(iodine-131, cobalt-60, etc.), particularly in the primary water contained
in micro-quantities by excluding disturbing factors to the measurement,
namely, the aforesaid annihilation gamma-rays emitted by other
radionuclides coexisting in the primary water, and Compton effects due to
the gamma-rays and annihilation gamma rays as far as possible.
Another object of this invention is to provide a high-sensitive measurement
method capable of detecting such extremely slight amounts of the
radionuclides emitting gamma rays in the primary water that it has been
not possible to detect hitherto.
Further object is to provide a reliable measurement method which enables
continuous surveillance of leakage of a nuclear fuel assembly, thereby
assisting in early prevention of the risk.
SUMMARY OF THE INVENTION
The invention for achieving the foregoing objects resides generally in a
method for measuring selectively gamma-rays of radionuclides of
microquantities, particularly in primary water of a nuclear reactor,
coexisting with radionuclides each emitting a pair of annihilation
gamma-rays in diametrically opposite directions, using a gamma-ray
spectrometric system which includes a primary detector for detecting
photons of the gamma-rays and photons of the one annihilation gamma-rays
in the one direction, a secondary detector for detecting at least photons
of the other annihilation gamma-rays in the opposite direction, a 10
shield detector for detecting photons of Compton-scattered gamma-rays
escaped from the primary detector to the shield detector, and an
anticoincidence circuit connecting with the primary, secondary, and shield
detectors, the primary detector and the secondary detector being located
in opposed manner relative to the axis of a coolant pipe, through which
the primary water flows, the shield detector being disposed to surround
the primary detector except for its portion facing to the pipe on which
the gamma-rays and the annihilation gamma-rays are incident.
The method comprises: detecting the photons of the gamma-rays and the
photons of the one annihilation gamma-rays on the primary detector as
pulses while detecting the photons of the other annihilation gamma-rays on
the secondary detector as pulses; and counting the pulses from the
secondary detector in anticoincidence with the pulses from the primary
detector thereby to reject the recording of the pulses of the annihilation
gamma-rays, thus minimizing the annihilation gamma-rays; and subsequently
measuring count numbers of the gamma-rays on the basis of the analysis of
the pulses.
More preferably, the method further comprises, simultaneously with the
foregoing detecting step, detecting the photons of the Compton-scattered
and escaped gamma-rays on the shield detector as pulses and counting the
pulses from the shield detector in anticoincidence with pulses from the
primary detector thereby to reject the recording of the pulses of the
Compton-scattered gamma-rays, thus additionally diminishing the Compton
gamma-rays.
According to the method of this invention, when the photons of the
annihilation gamma-rays emitted in diametrically opposite directions are
coincidently detected on the primary and secondary detectors located in an
opposed relation to each other, the resulting pulses are rejected by
anticoincidence counting operation of the anticoincidence circuit, whereby
the annihilation gamma-rays are vastly reduced from the primary detector.
Consequently, it is possible to elevate significantly the detection limits
of the intended gamma-rays of iodine-131, cobalt-60, etc.
Further according to a preferred embodiment, when the photons of
Compton-scatterered gamma-rays are coincidently detected on the primary
and shield detectors, the resulting pulses are rejected by anticoincidence
counting operation of the anticoincidence circuit, whereby the Compton
gamma-rays are also significantly diminished from the primary detector,
which enables to further enhance the detection limits of the intended
gamma-rays of radionuclides.
BRIEF DESCRIPTION OF THE DRAWINGS
Examples of the invention will be hereinbelow described in more detail with
reference to the accompanying drawings; in which,
FIG. 1 is a schematic longitudinal sectional view of one example of a
gamma-ray spectrometric measurement system for carrying the method of this
invention into effect showing only a detecting device as an essential part
of the system;
FIG. 2 is a schematic transverse sectional view of the detecting device in
FIG. 1; and
FIG. 3 is a diagrammatic illustration of a gamma-ray spectrometric
measurement system used for this invention showing its essential elements.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring to FIGS. 1 and 2, a detecting device is shown, which comprises a
primary detector 1 and a secondary detector 2 located in mutually opposed
manner relative to the axis of a pipe P, e.g., a coolant pipe, through
which for example, water of a non-reproductive cooler (not shown) flows,
and a shield detector 3 surrounding substantially the whole perimeter of
the primary detector 1 except for its portion facing the pipe.
The primary detector 1 is constructed preferably of a semiconductor
detector or a scintillation detector.
The secondary detector 2 and the shield detector 3 are constructed each of
one or more scintillation detectors.
The semiconductor detector to be used includes, for example, HP Ge
(high-purity germanium) detector, Si(Li) (lithium drift silicon) detector,
CdTe (cadmium telluride) detector, GaAs (gallium arsenide) detector,
HgI.sub.2 (mercuric iodide) detector, etc.
The scintillation detector usable for this invention includes, for example,
NaI(T1) (sodium iodide activated by tallium) detector, CsI(T1) (cesium
iodide activated by thallium) detector, Bi.sub.4 Ge.sub.3 O.sub.12
(bismuth germanate known as BGO) detector, or the like.
The secondary detector 2 is configured in a sector-form so as to surround a
half to one-third of the periphery of the coolant pipe P.
The primary, secondary, and shield detectors 1,2,3 are surrounded by lead
shields 4 except for their directions in which gamma-rays and annihilation
gamma-rays in the primary water are incoming and detected so that the
incident gamma-rays and annihilation gamma-rays may be collimated and the
incident dose of the gamma-rays and annihilation gamma-rays may be
restricted.
The detecting device including the primary detector 1, the secondary
detector 2 and the shield detector 3 is connected with an anticoincidence
circuit 11 for operating an anticoincidence counting between the primary
and secondary detectors 1,2 and between the primary and shield detectors
1,3, and a multichannel pulse height analyzer 12, thus forming a gamma-ray
spectrometric system as a whole, as shown in FIG. 3 in which essential
components only are depicted with other components omitted since they are
well-known per se in the art.
In the gamma-ray spectrometry system thus constructed, the primary detector
1 serves to detect photons of gamma-rays from .sub.131 I, .sub.60 Co, and
the like as intended as well as photons of the one annihilation gamma-rays
as pulses; the secondary detector 2 serves to detect photons of the other
annihilation gamma-rays as pulses; and the shield detector 3 serves to
detect photons of the gamma-rays, which are Compton-scattered and escaped
from the primary to the shield detectors, as pulses.
When the photons of the gamma-rays or annihilation gamma-rays are detected
on a germanium detector, an interaction of the photons with the germanium
material yields gamma-ray spectra which usually include each a
photoelectric peak due to full energy absorption event and a continuous
sepctrum called Compton continuum or background due to once or twice
scattering and subsequent escaping of the scattered photons outside the
detector. Thus, the full energy absorption event is never attended with
the escaping of scattered photons.
The photopeak is utilized to determine the gamma-ray energy which is
important for identification and quantitative determination of a
radionuclide whereas the Compton backgound is a disturbing factor for the
gamma-ray spectrometric measurement.
Consequently, when the escaped photons are detected coincidently by means
of the shield detector 3 and the primary detector 1 and an anticoincidence
counting is operated, that tends to reject selectively the Compton
scattering events only without affecting the full energy events.
Further simultaneously when the annihilation gamma-rays are coincidently
detected on the primary detector 1 and the secondary detector 2, and an
anticoincidence counting is operated, that assists in rejecting
selectively the events due to the annihilation gamma-rays without
affecting the full energy events.
More specifically, the rejection is conducted by passing the pulses from
the primary detector 1 through electron gates of the anticoincidence
circuit 11 which are adapted to be closed when pulses are detected on the
secondary detector 2 and the shield detector 3, coincident with the
detection on the primary detector 1.
The rejection by anticoincidence counting operation yields the result that
the annihilation gamma-rays and Compton background gamma-rays are
significantly reduced, which enables it to increase the
photopeak-to-background ratio in the spectrum and accordingly, to
determine the count numbers of the intended gamma-rays with more
precision.
In the multichannel pulse height analyzer 12, the pulses detected by
conversion of the radiation energy to voltage or current in proportion to
the energy are divided into thousands of intervals (namely, multichannels)
over the whole voltage or current pulse range, and number ratios of the
pulses belonging to the respective channels are determined, thus yielding
an energy distribution of the gamma-rays, i.e. gamma-spectra, from which
count numbers of the intended gamma-rays are determined.
One example of a method of the invention will be explained when applied to
primary water of a nuclear reactor, i.e. non-reproductive cooler by
fitting a coolant pipe P connecting to the cooler on its inflow side with
the detecting device as described above including the primary, secondary,
and shield detectors 1,2,3.
As the primary detector 1, a germanium detector was used, which had a good
energy resolution having a half band width of up to 2.0 KeV when 1.33 MeV
gamma-ray of .sup.60 Co was taken as a standard and a counting efficiency
of at least 75%.
The shield detector used has such dimensions that make the Compton
background (continuum) in the 131I area of the spectrum smaller than 1/10
of that without Compton suppression.
When pulses from the primary detector 1 were counted in anticoincidence
with pulses from the secondary detector 2 and the shield detector 3 by the
operation of the anticoincidence circuit 11, the annihilation gamma-rays
and Compton gamma-rays could be significantly diminished.
Then, count numbers of the gamma-rays from .sup.131 I, .sup.60 Co, and
others in the primary detector 1 were determined from the resulting
gamma-spectra by analysis with the multi-channel pulse height analyzer 12.
As a result, the detection limit of I gamma-ray area was enhanced to less
than 1.5 Bq/cm.sup.3, more than 10 times as high as that (15 Bq/cm.sup.3)
of a conventional method without reduction of the annihilation gamma-rays.
This invention has been so far described, by way of example, with a primary
water of a nuclear reactor, but the method can be naturally used for the
analysis of: steam of a secondary system (from a steam generator), drain
water of a primary coolant, chemical analysis of a primary coolant, etc.
in the nuclear power field.
However, this invention is also applicable to other fields, namely,
researches in high energy physics, micro-analysis in accelerator
engineering, etc.
As described above, the conspicuous elevation of the detection limits of
gamma-rays makes it possible to conduct continuous measurement of highly
low-concentrations of radionuclides in primary water of a nuclear reactor,
with the result that security of the nuclear reactor can be ensured by
early detection of leakage of the fuel assembly. Further, it is possible
to decrease the frequency of chemical analysis for detecting the leakage
which has been hitherto performed in nuclear power plants.
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